Tweak the name for the function that diagnoses when fuzzing external libraries (#5974)

The old function name caused some confusion during the review of #5338,
sending this to see if it provides a less surprising function name and
boolean result. Happy to try other names / approaches as well.

toolchain/driver/lld_subcommand.cpp

@@ -95,7 +95,7 @@ auto LldSubcommand::Run(DriverEnv& driver_env) -> DriverResult {
 
   // Don't run LLD when fuzzing, as we're not currently in a good position to
   // debug and fix fuzzer-found bugs within LLD.
-  if (!DisableFuzzingExternalLibraries(driver_env, "lld")) {
+  if (TestAndDiagnoseIfFuzzingExternalLibraries(driver_env, "lld")) {
     return {.success = false};
   }